JP2007077496A - Ferritic stainless-steel sheet with excellent corrosion resistance and process for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless-steel sheet with excellent corrosion resistance, and to provide a process for producing the same. <P>SOLUTION: The ferritic stainless-steel sheet contains up to 0.03% C, up to 1.0% Si, up to 0.5% Mn, up to 0.04% P, up to 0.02% S, up to 0.1% Al, 20.5 to 22.5% Cr, 0.3 to 0.8% Cu, up to 1.0% Ni, 4×(C%+N%) to 0.35% Ti, up to 0.01% Nb, and up to 0.03% N, provided that C+N is up to 0.05% and the remainder is Fe and unavoidable impurities. It satisfies the relationship 240+35×(Cr%-20.5)+280×äTi%-4×(C%+N%)}≥280. Further, upon the production of the steel sheet, a stainless steel having a composition containing the above and satisfying the above inequality as the stock is subjected to hot rolling, and the hot rolled sheet is subjected to continuous annealing at 800 to 1,000°C, is thereafter pickled, and is subsequently subjected to the stages of cold rolling, finish annealing, cooling and pickling, so as to be a cold rolled-annealed sheet. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a ferritic stainless steel sheet having excellent corrosion resistance and a method for producing the same.

  Among stainless steels, austenitic stainless steel SUS304 (18% Cr-8% Ni) (Japanese Industrial Standard, JIS G 4305) is widely used due to its excellent corrosion resistance. However, this steel type is expensive because it contains a large amount of Ni. On the other hand, as a ferritic stainless steel not containing a large amount of Ni, there is SUS436L (18% Cr-1% Mo) (JIS G 4305) to which Mo is added as a steel type having excellent corrosion resistance equivalent to SUS304. However, this is also an element where Mo is expensive, so even if only 1% of Mo is added, the cost is greatly increased.

  From the above situation, ferritic stainless steel having corrosion resistance equivalent to SUS304 or SUS436L is required without adding Mo. There is SUS430J1L (19% Cr-0.5% Cu-0.4% Nb) (JIS G 4305) as a ferritic stainless steel not containing Mo, but its corrosion resistance is inferior compared to SUS304 and SUS436L.

On the other hand, in Patent Document 1, the component composition is characterized by Cr: 9-30%, Cu: 0.1-0.6%, Ti: 5 × C% -15 × C%, Sb: 0.02-0.2% In ferritic stainless steel, in Patent Document 2, the component composition is Cr: 11-23%, Cu: 0.5-2.0%, at least one of Ti, Nb, Zr, Ta is 0.01-1.0%, V: 0.05 Ferritic stainless steel characterized by ~ 2.0% is further disclosed in Patent Document 3 as component composition: Cr: 5-60%, Cu: 0.15-3.0%, Ti: 4 x (C% + N%) -0.5 % And Nb: 0.003 to 0.020%, respectively, are disclosed.
Japanese Patent Publication No. 50-6167 Japanese Patent Publication No.64-4576 Japanese Patent No. 3420371

  However, Patent Documents 1 to 3 do not show components that achieve both high-efficiency productivity by continuous annealing of hot-rolled sheets and high-speed continuous annealing of cold-rolled sheets and excellent corrosion resistance equivalent to SUS304 or SUS436L.

  In order to manufacture at low cost, it is necessary not to add expensive Mo and to be able to mass-produce with higher efficiency. Increasing the Cr content improves the corrosion resistance, but decreases the toughness of the hot-rolled sheet. Hot-rolled sheets of high Cr ferritic stainless steel need to be annealed and pickled in a continuous annealing / pickling line before cold rolling, but if the toughness of the hot-rolled sheet is low, the continuous annealing / pickling line In some cases, it is not possible to pass through. In addition, from the viewpoint of high-efficiency productivity, it is also necessary to be able to efficiently anneal cold-rolled sheets in a high-speed continuous annealing line for cold-rolled sheets that are also used as ordinary steel.

  In view of such circumstances, an object of the present invention is to provide a ferritic stainless steel plate that is inexpensive and highly efficient and has excellent corrosion resistance, and a method for manufacturing the same.

  In order to solve the above-mentioned problems, the present inventors diligently studied on a method for obtaining a stainless steel plate that does not contain expensive Ni and Mo and has excellent corrosion resistance. As a result, Cr is limited to a range of 20.5% to 22.5% from the viewpoint of corrosion resistance and manufacturability, and carbon and nitrogen as impurity elements are reduced. Furthermore, by adding an appropriate amount of Ti, SUS304 or We found that stainless steel sheets with excellent corrosion resistance equivalent to SUS436L can be obtained, and that high-efficiency production is possible by performing continuous annealing of hot-rolled sheets and annealing of cold-rolled sheets in a high-speed continuous annealing line for cold-rolled sheets. It was.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] In mass%, C: 0.03% or less, Si: 1.0% or less, Mn: 0.5% or less, P: 0.04% or less, S: 0.02% or less, Al: 0.1% or less, Cr: 20.5% or more, 22.5 % Or less, Cu: 0.3% or more, 0.8% or less, Ni: 1.0% or less, Ti: 4 × (C% + N%) or more, 0.35% or less, Nb: 0.01% or less, N: 0.03% or less, C + N: A ferritic stainless steel sheet excellent in corrosion resistance, characterized by containing 0.05% or less, the balance being Fe and inevitable impurities and satisfying the following formula (1).
240 + 35 × (Cr% −20.5) + 280 × {Ti% −4 × (C% + N%)} ≧ 280 (1)
Here, C%, N%, Cr%, and Ti% represent the contents (mass%) of C, N, Cr, and Ti, respectively.
[2] In mass%, C: 0.03% or less, Si: 1.0% or less, Mn: 0.5% or less, P: 0.04% or less, S: 0.02% or less, Al: 0.1% or less, Cr: 20.5% or more, 22.5 % Or less, Cu: 0.3% or more, 0.8% or less, Ni: 1.0% or less, Ti: 4 × (C% + N%) or more, 0.35% or less, Nb: 0.01% or less, N: 0.03% or less, C + N: 0.05% or less, with the balance being Fe and inevitable impurities, and using stainless steel having a composition that satisfies the following formula (1) as a raw material, the raw material is hot-rolled to 800 to 1000 ° C. Ferritic steel with excellent corrosion resistance, characterized by continuous annealing of hot-rolled sheets at a temperature of, followed by pickling, followed by cold rolling, finish annealing, cooling, and pickling processes to form cold-rolled annealed sheets Manufacturing method of stainless steel sheet.
240 + 35 × (Cr% −20.5) + 280 × {Ti% −4 × (C% + N%)} ≧ 280 (1)
Here, C%, N%, Cr%, and Ti% represent the contents (mass%) of C, N, Cr, and Ti, respectively.
In addition, in this specification, all% which shows the component of steel is mass%.

  According to the present invention, a ferritic stainless steel sheet having excellent corrosion resistance corresponding to SUS304 or SUS436L can be obtained without adding expensive Mo or the like. Further, the stainless steel plate of the present invention can be produced at low cost because it can be produced with high efficiency and does not contain expensive Ni or Mo.

  Furthermore, since the stainless steel plate of the present invention is added with Ti, which is a stabilizing element that reduces impurity elements and fixes C and N in the steel, it also has weldability, weld zone workability, and weld zone corrosion resistance. Excellent.

  The present invention is described in detail below. First, the component composition of the present invention will be described.

C: 0.03% or less, N: 0.03% or less, C + N: 0.05% or less
Since C and N reduce the toughness of the hot-rolled sheet, it is desirable that they be less, and each is limited to 0.03% or less, and the total (C + N) is limited to 0.05% or less. Preferably, C: 0.015% or less, N: 0.015% or less, and C + N: 0.03% or less.

Si: 1.0% or less
Si is an element necessary as a deoxidizer. However, if added in a large amount, the toughness of the hot-rolled sheet is lowered. Therefore, Si is 1.0% or less. Preferably, it is 0.3% or less.

Mn: 0.5% or less
Mn has a deoxidizing action. However, it is desirable that the addition amount be low because sulfides are formed in the steel and the corrosion resistance is remarkably reduced. In consideration of economics during production, Mn is set to 0.5% or less. Preferably, it is 0.3% or less.

P: 0.04% or less
P is preferably small in terms of hot workability, and P is 0.04% or less.

S: 0.02% or less
S is preferably smaller in terms of hot workability and corrosion resistance, and S is 0.02% or less. Preferably, it is 0.005% or less.

Al: 0.1% or less
Al is an effective component for deoxidation. However, excessive addition causes surface flaws due to an increase in Al-based non-metallic inclusions and decreases workability. Therefore, Al is made 0.1% or less.

Cr: 20.5% or more, 22.5% or less
Cr is the most important component in the present invention. It is effective for improving corrosion resistance, and in order to obtain corrosion resistance equivalent to SUS304 or SUS436L, addition of 20.5% or more is necessary. On the other hand, if added over 22.5%, the toughness of the hot-rolled sheet is lowered, and continuous annealing of the hot-rolled sheet becomes difficult. Therefore, Cr is 20.5% or more and 22.5% or less. Preferably, it is 20.5 to 21.5%.

Cu: 0.3% or more, 0.8% or less
Cu is an important component in the present invention. It is an element necessary for reducing crevice corrosion. For that purpose, it is necessary to add at least 0.3% or more. On the other hand, when it exceeds 0.8%, hot workability deteriorates. Therefore, Cu is 0.3% or more and 0.8% or less. Preferably, it is 0.3% or more and less than 0.5%.

Ni: 1.0% or less
Ni has an effect of preventing deterioration of hot workability due to addition of Cu. However, in addition to being an expensive element, the effect is saturated even if added over 1.0%. Therefore, Ni is 1.0% or less. Preferably, it is 0.1 to 0.4%.

Ti: 4 × (C% + N%) or more, 0.35% or less
Ti is the most important component in the present invention. It is an additive element that is the basis of the present invention, and is an element that needs to be added in order to obtain excellent corrosion resistance equivalent to SUS304 or SUS436L with a Cr addition amount of up to 22.5%. Ti has traditionally been recognized as an element that has the effect of detoxifying C and N, which are harmful to the workability and corrosion resistance of welds, as TiC and TiN to improve corrosion resistance, but in the present invention Ti is directly pitting corrosion. It has been found that it has the effect of increasing the potential and improving the corrosion resistance. Furthermore, Ti is added to prevent sensitization by continuous annealing. In order to obtain the above effects, it is necessary to add 4 × (C% + N%) or more. On the other hand, if it exceeds 0.35% and is added excessively, the toughness of the hot rolled sheet is deteriorated. Therefore, Ti is 4 × (C% + N%) or more and 0.35% or less. Preferably, 8 × (C% + N%)} to 0.30%.

Nb: 0.01% or less
Nb raises the recrystallization temperature, and in the high-speed cold-rolled sheet annealing line, annealing becomes insufficient and workability cannot be secured. Therefore, Nb is made 0.01% or less. Preferably, it is 0.005% or less.

240 + 35 × (Cr% −20.5) + 280 × {Ti% −4 × (C% + N%)} ≧ 280
In the present invention, Cr, Ti, C and N satisfy the relationship of the formula (1) in order to obtain excellent corrosion resistance equivalent to or higher than SUS304 or SUS436L without containing Ni or Mo.
240 + 35 × (Cr% −20.5) + 280 × {Ti% −4 × (C% + N%)} ≧ 280 Formula (1)
Cr and Ti have the effect of increasing the pitting corrosion potential, but in order to obtain excellent corrosion resistance equivalent to SUS304 or SUS436L or more, just add Cr 20.5% or more and Ti 4 × (C% + N%) or more. Is insufficient, and further, the Cr content and the Ti content must satisfy the above formula (1) in consideration of the C content and the N content. Equation (1) is derived from the relationship between the Cr content and Ti content and the pitting potential (mV vs SCE). The pitting potential value is 280 mV, which is a typical pitting potential value for SUS304 or SUS436L. The lower limit values of the Cr content and the Ti content are as described above. In addition, Ti, except for those bonded as TiC and TiN, exhibits the effect of improving the pitting potential, so that Ti corresponding to the amount of dissolved Ti {Ti% -4 × ( C% + N%)} is used in equation (1).

Mo: 0.2% or less
Mo is an element that improves corrosion resistance. In addition to being an expensive element, Mo lowers the toughness of hot-rolled sheets, making it difficult to manufacture, and hardens cold-rolled annealed sheets to reduce workability. Therefore, it should be 0.2% or less. Preferably, it is 0.1% or less.
In addition, the following elements can be added as needed.
B: 0.0002% or more, 0.002% or less
B is an element effective for improving secondary work embrittlement resistance after deep drawing. The effect cannot be obtained at less than 0.0002%. On the other hand, excessive addition deteriorates hot workability and deep drawability. Therefore, when adding, the addition amount is preferably 0.0002% or more and 0.002% or less.

  V: 0.01% or more, 0.5% or less, Zr: 0.01% or more, 0.5% or less V and Zr have the effect of detoxifying C and N and preventing intergranular corrosion from occurring in the weld. The effect does not appear when V and Zr are each less than 0.005%, and addition of 0.01% or more is necessary. However, if V and Zr are added in excess of 0.5%, the toughness of the hot-rolled sheet is lowered and manufacturing becomes difficult. Furthermore, V and Zr combine with C, N or O to generate inclusions and increase surface defects. Therefore, when added, V and Zr are preferably 0.01% or more and 0.5% or less, respectively.

  The balance other than the above is Fe and inevitable impurities.

Next, the manufacturing method of the ferritic stainless steel plate excellent in corrosion resistance of the present invention will be described.
As a highly efficient manufacturing method of the steel of the present invention, it is continuously cast on a slab, heated to 1100 to 1250 ° C. and hot-rolled to form a hot-rolled coil, and this is used in a continuous annealing / pickling line for hot-rolled sheets. Annealing and acid pickling at a temperature of ~ 1000 ° C, then cold rolling and cold rolling to efficiently cool and roll the cold-rolled sheet in a high-speed continuous annealing line for cold-rolled steel combined with ordinary steel A method of washing is recommended.
Details are as follows.
First, molten steel adjusted to the above chemical composition range is melted by secondary refining using a converter, electric furnace, etc. and strong stirring, vacuum oxygen decarburization (VOD) or argon / oxygen decarburization (AOD). . Next, the slab is melted from the molten steel by continuous casting or ingot forming. The casting method is preferably continuous casting in terms of productivity and quality.
The slab obtained by casting is reheated to 1100 to 1250 ° C if necessary, hot-rolled to a thickness of 2.0 to 6.0 mm, and continuously annealed at a temperature of 800 to 1000 ° C and then acid. Wash. If it is less than 800 ° C, strain due to rolling remains and becomes hard, so surface scratches are likely to occur, and if it exceeds 1000 ° C, it tends to coarsen and the toughness decreases, so the temperature range of continuous annealing of hot-rolled sheet is 800 ~ Set to 1000 ° C.
The pickled hot-rolled sheet is subjected to cold rolling, finish annealing, cooling, and pickling in order to obtain a cold-rolled annealed sheet having a thickness of 0.03 to 5.0 mm.
The rolling reduction at the time of cold rolling is preferably 25% or more in order to ensure the mechanical properties such as toughness and workability aimed by the present invention. More preferably, it is 50% or more. Further, the cold rolling may be one or two or more cold rolling including intermediate annealing. The steps of cold rolling, finish annealing, and pickling may be repeated. Furthermore, an efficient cold-rolled sheet annealing and pickling method is recommended in a high-speed continuous annealing line for cold-rolled sheets that are also used as ordinary steel. Moreover, although productivity falls, you may perform cold-rolled sheet annealing and pickling in a general stainless steel cold-rolled sheet annealing and pickling line. Moreover, you may perform bright annealing by a bright annealing line as needed. When welding the steel plate of the present invention described above, all ordinary welding methods such as arc welding including TIG and MIG, resistance welding such as seam welding, spot welding, and laser welding, and laser welding can be applied.

  A ferritic stainless steel having the composition shown in Table 1 (partially austenitic stainless steel in the comparative example) is melted in a 30 kg steel ingot, heated to a temperature of 1150 ° C. and hot-rolled to a thickness of 2.5 A hot rolled sheet of ˜2.8 mm was obtained. Here, the amount of Mo added is adjusted to an amount that is expected to be mixed as an impurity in the actual process. A test piece (JIS B 7722 V notch) was collected in the rolling direction on the obtained hot rolled sheet and subjected to a Charpy impact test. In Comparative Example 11 where Cr is 22.8%, which is high outside the scope of the present invention, and Comparative Example 12 where Ti is 0.39%, which is high outside the scope of the present invention, it is difficult to continuously anneal hot-rolled sheets in actual processes because of low toughness. No further tests were conducted.

  The remaining specimens except Comparative Examples 11 and 12 were subjected to continuous annealing at 950 ° C., pickling, and cold rolling to produce cold rolled sheets having a thickness of 0.8 mm. Next, finish annealing, cooling, and pickling were performed at 880 ° C. in an air atmosphere. In Comparative Example 13 where Nb is 0.15%, which is high outside the scope of the present invention, annealing is insufficient at this temperature and the elongation is less than 20%, and cold rolling annealing in a high-speed continuous annealing line for cold rolling is sufficient. Since subsequent processability could not be ensured, no subsequent tests were conducted.

  The remaining specimens (Examples 1 to 8 and 21 to 25, Comparative Examples 14 to 16) excluding Comparative Examples 11 to 13 obtained as described above and 0.8 mm thick cold rolled annealing of SUS304, SUS436L and SUS430J1L The test piece collected from the plate was subjected to a salt spray cycle test while measuring the pitting potential in a 3.5% NaCl solution at 30 ° C. according to JIS G 0577. In the salt spray cycle test, salt water spray (5% NaCl, 35 ° C, spray 2h) → dry (60 ° C, 4h, relative) to the test material (20mm x 30mm) whose surface was polished with No. 600 abrasive paper The humidity was 40%) → wet (50 ° C., 2 hours, relative humidity ≧ 95%), and 45 cycles were performed. The obtained results are also shown in Table 1.

  Next, crevice corrosion tests were performed on the remaining test materials (Invention Examples 1-8 and 21-25, Comparative Example 16) excluding Comparative Examples 11-15 and SUS430J1L, SUS304, and SUS436L. In this test, 60 mm wide x 80 mm wide and 20 mm wide x 30 mm long flat plates taken from each test material were used, and their surfaces were polished with No. 600 abrasive paper, then 60 mm wide x 80 mm long flat plates. A flat plate having a width of 20 mm and a height of 30 mm was placed thereon so that the respective diagonal lines overlapped, and a center structure was joined by spot welding to form a gap structure. The test piece was subjected to the above-described salt spray cycle test for 90 cycles, the spot weld was removed, the gap was opened, and the depth of the corrosion hole was measured with a laser microscope. The results obtained above are also shown in Table 1.

In Table 1, the criteria for each test are as follows.
(1) Charpy impact test: Absorbed energy at 25 ° C of 50 J / cm ² or more was judged as ○ (pass), and less than 50 J / cm ² was judged as × (fail).
(2) Cold-rolled sheet annealing: 20% or more of the elongation after annealing at 880 ° C. was judged as “good” (accepted), and the elongation after annealing at 880 ° C. was judged less than 20% as “x” (failed).
(3) Salt spray cycle test: With respect to one side (60 mm × 80 mm) of the test piece, the rusting area was determined to be less than 20% (good) and 20% or more as x (failed).
(4) Crevice corrosion test result: Of the corrosion holes generated in the crevice part of the test piece, the average value of 10 deep points was determined to be ○ (pass) when less than 300 μm, and × (fail) when 300 μm or more. The depth of the corrosion hole was measured with a laser microscope. From Table 1, it can be seen that in the present invention example, the pitting potential is equal to or higher than that of SUS304 and SUS436L, the result of the salt spray cycle test is also good, and the corrosion resistance is excellent. Further, the average depth of the corrosion holes in the crevice corrosion test is less than 300 μm, and the crevice corrosion resistance is excellent.

  On the other hand, the pitting corrosion potential of Comparative Example 14 which is lower than Cr and 20.1% outside the scope of the present invention and Comparative Example 15 which does not satisfy the formula (1) is lower than that of SUS304 or SUS436L. Was inferior.

  FIG. 1 shows the relationship between the pitting potential, Cr%, and Ti% −4 × (C% + N%) for Invention Examples 1 to 8, 21 to 25, and Comparative Examples 14, 15, and 16. From Fig. 1, in order to obtain a pitting corrosion potential of 280 mV or more equivalent to SUS304 or SUS436L, the formula (1) 240 + 35 × (Cr% −20.5) + 280 × {Ti% −4 × (C% + N%) It is clear that it is necessary to satisfy)} ≧ 280.

  In addition, Comparative Example 16 to which Cu was not added has an average depth of the corrosion holes in the crevice corrosion test of 300 μm or more, and crevice corrosion resistance compared to Invention Examples 1 to 8, 21 to 25, SUS304, and SUS436L. The sex is inferior.

  From the above, in the example of the present invention, the hot-rolled sheet can be continuously annealed, and the cold-rolled sheet can be annealed in the high-speed continuous annealing line of the cold-rolled sheet with an elongation of 20% or more at 880 ° C, enabling high-efficiency production. I found out that It was also revealed that it has excellent corrosion resistance equivalent to SUS304 or SUS436L.

  It is suitable as a member that requires corrosion resistance, mainly for maritime shipping containers, containers, kitchen equipment, building interior / exterior materials, automobile parts, elevators, escalators, railway vehicles, outer casings of electrical equipment casings, and the like.

It is a figure which shows the relationship between a pitting corrosion potential, Cr%, and Ti% -4x (C% + N%).

Claims (2)

In mass%, C: 0.03% or less, Si: 1.0% or less, Mn: 0.5% or less, P: 0.04% or less, S: 0.02% or less, Al: 0.1% or less, Cr: 20.5% or more, 22.5% or less, Cu: 0.3% or more, 0.8% or less, Ni: 1.0% or less, Ti: 4 × (C% + N%) or more, 0.35% or less, Nb: 0.01% or less, N: 0.03% or less, C + N: 0.05 % Ferritic stainless steel sheet with excellent corrosion resistance, characterized in that the balance is composed of Fe and inevitable impurities, and satisfies the following formula (1).
240 + 35 × (Cr% −20.5) + 280 × {Ti% −4 × (C% + N%)} ≧ 280 (1)
Here, C%, N%, Cr%, and Ti% represent the contents (mass%) of C, N, Cr, and Ti, respectively. In mass%, C: 0.03% or less, Si: 1.0% or less, Mn: 0.5% or less, P: 0.04% or less, S: 0.02% or less, Al: 0.1% or less, Cr: 20.5% or more, 22.5% or less, Cu: 0.3% or more, 0.8% or less, Ni: 1.0% or less, Ti: 4 × (C% + N%) or more, 0.35% or less, Nb: 0.01% or less, N: 0.03% or less, C + N: 0.05 %, With the balance consisting of Fe and inevitable impurities, and stainless steel having a composition that satisfies the following formula (1) as a raw material, the raw material is hot-rolled at a temperature of 800 to 1000 ° C. A hot-rolled sheet is continuously annealed and then pickled, followed by cold rolling, finish annealing, cooling, pickling, and cold-rolled annealed sheet. Production method. 240 + 35 × (Cr% −20.5) + 280 × {Ti% −4 × (C% + N%)} ≧ 280 (1)
Here, C%, N%, Cr%, and Ti% represent the contents (mass%) of C, N, Cr, and Ti, respectively.

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